Since $P$ is the force which is just enough to start the motion of the body,
$P =\mu_{ s } N =\mu_{ s }\,mg$
When the body moves under this force, the net force on body is given by
$F=m a=P-f$
$\Rightarrow ma =\mu_{ s } mg -\mu_{ k } N$
$= m \left(\mu_{ s }-\mu_{ k }\right) g$
$\Rightarrow a =\left(\mu_{ s }-\mu_{ k }\right) g$
$=(0.5-0.4)(10)\,m / s ^2$
$\Rightarrow a =1\,m / s ^2$





